1. Field of the Invention
This invention relates to moisture control in a material handling system, and more particularly to an apparatus and method used to control the moisture content of solid particulate material, such as plastic pellets used for making thermoplastic molded articles and extrusions.
2. Brief Description of the Prior Art
A typical system for controlling the moisture content in a particulate bed, for example, supplies moisture-controlled air to the particulate bed and uses a dessicant to dehumidify air returning from the particulate bed.
This system causes very low dew point air to be supplied to the particulate bed. When the material in the particulate bed comprises resins such as certain homopolymer and copolymer polyamides and some polyesters (e.g., nylon 6 and 6.6), the resins may be over-dried undesirably. Some moisture content is required to maintain desired impact resistance and flexibility in finished products produced from the resins.
A prior approach to the over-drying problem proposed to sense the moisture content of air being supplied to a particulate bed and to inject moisture-containing compressed air into the particulate bed as necessary. But this approach has certain disadvantages.
Therefore, a need has continued to exist for a method and apparatus for accurately maintaining a desired level of moisture in a particulate bed by removing unwanted, excessive moisture from the particulate bed without lowering the moisture content to an undesirable level.